1. Field of the Invention
This invention relates to interferometry and more particularly to an autofocus system using a common path interferometer.
2. Description of the Prior Art
Autofocus systems are well known, for instance in the photography field. Typically an autofocus system includes a range finding device to determine the distance from the lens system to the object to be imaged. For instance, some autofocus systems use infrared range finding or ultrasonic range finding. In addition to the photography field, autofocus is also used in the lithography field where the goal is to form an image on a resist coated semiconductor wafer. In this case typically the focus adjustment is the distance between the projection lens system of a photolithography tool and the image plane which is the surface of the resist formed on a wafer. Photolithography as used in the semiconductor field is extremely precise and the autofocus system must correspondingly be extremely precise since the feature sizes being imaged are typically very small, for instance 0.25 micrometers. It is known to use an oblique incident optical system for lithography auto focusing, where displacement of a light beam is measured by a sensor. It is to be understood that "in focus" in this context means that the image plane is at a particular distance from the projection optical system.
However, it has been found that in general vibrations and random air fluctuations are still a problem in an autofocus system using oblique incident optics. In general, compensation for this vibration is not possible, leading to a degradation of the accuracy of the autofocusing.